1. Field of the Invention
The present invention relates to a method and apparatus for exposing an optical disk master.
2. Description of the Related Art
With a growing amount of information needed for information processing equipment and image/acoustic equipment, optical disks have attracted considerable attention in recent years because they are excellent in easy data access, mass storage, and miniaturization of the equipment. There are three types of optical disks: read-only type, write-once type, and recording/reproducing type. Examples of the recording/reproducing type include a phase change optical disk and a magneto-optical disk. These disks are called CD, DVD, MO, or the like and are used in various applications.
For a read-only optical disk, embossed pit trains are formed in spiral fashion, and information is reproduced by irradiating these pit trains with reproduction light and detecting the presence or absence of a pit and the pit length from a change in intensity of the reflected light.
For a recording/reproducing optical disk, grooves or grooves and pits are formed in spiral fashion. Addresses are formed in some way on the grooves or the grooves and pits. The addresses are used to determine the position for recording/reproducing. There are two recording systems for the recording/reproducing optical disk: one system allows information to be recorded on either grooves or lands, and the other system allows information to be recorded on both grooves and lands. The groove width and the space between the grooves differ depending on the recording system. Recording or reproduction is performed by forming a recording mark in a recording layer provided on the grooves or the lands. In the case of a phase change optical disk, e.g., a phase change recording layer is used as the recording layer. The phase change recording layer is made of a material whose refractive index with respect to the recording/reproducing light varies according to phase. The phase change can be achieved in such a manner that the time and intensity of irradiation during recording/reproducing are varied to cause a change in cooling rate of the recording layer. When the recording/reproducing optical disk includes the phase change recording layer, the phase change recording layer can be formed, e.g., in the grooves, and information is recorded/reproduced by forming marks with different phases in the phase change recording layer.
The capacity of optical disks increases steadily from about 650 MB (CD) to about 5 GB (DVD) with a rise in the market demand and the amount of information. In particular, the recording/reproducing optical disks are expected to substitute for tape media, and DVD-RAM already has achieved a capacity of about 5 GB. The capacity of about 5 GB enables recording for approximately two hours at an average transfer rate of about 5 Mbps. The average transfer rate of about 5 Mbps corresponds substantially to normal quality images in a compressed format called MPEG2. The market now demands optical disks that can record images with higher quality and definition for approximately two hours. The capacity can be increased, e.g., by forming multi-layer optical disks, improving recording areal density, and enhancing format efficiency. Concerning the recording areal density, DVD-RAM has attained a track pitch of about 0.615 μm and a minimum mark length of 0.42 μm.
An improvement in areal density for large capacity requires even narrower track pitch and shorter mark length. To record images with high quality and high definition for two or more hours, a capacity of about 25 GB is necessary, which is five times as large as the DVD. Therefore, the spot size of a beam for reproduction should be reduced to 0.44 times or less. Moreover, a minimum pit length of about 0.18 μm or less and a track pitch of about 0.35 μm or less in terms of DVD are needed to follow the reduced spot size.
The amplitude of a push-pull tracking error signal decreases with a decrease in track pitch. Therefore, when the track pitch becomes narrower as described above, the push-pull tracking error signal decreases sharply. This makes it difficult to perform tracking servo. Even if the tracking servo is performed, it is prone to deviate from its proper operation. Thus, strict track pitch accuracy is essential for a decrease in track pitch.
FIG. 8 shows a conventional optical disk master exposing apparatus. A substrate disk 102 made of glass and coated with a photoresist 101 is exposed so as to produce an optical disk master. The substrate disk 102 is supported by a turntable 104 having a rotation control mechanism 103.
A light beam 106 emitted from a laser source 105 is incident on a power regulator 107, where the power of the laser beam 106 is adjusted. Then, a deflector 110 including an acoustooptical element deflects the laser beam 106 based on a deflection signal 109 output from a deflection signal source 108. The deflection is performed with the deflector 110 to form wobbled grooves for generating a reproduction clock. The deflected laser beam 106 is incident on an expander 116, where the beam diameter is expanded, passes through mirrors 120a, 120b, and 120c, and is focused on the photoresist 101 on the substrate disk 102 through an objective lens 117. The objective lens 117 is transferred in the radial direction of the substrate disk 102 by a transfer mechanism 118. The distance between the objective lens 117 and the substrate disk 102 is kept constant by a focusing control mechanism 119.
In this apparatus, the laser beam 106 fluctuates due to the optical elements arranged on the optical path of the laser beam 106, which degrades the pitch accuracy of tracks formed on the substrate disk 102. Moreover, there is also a limit to the track pitch accuracy because the transfer accuracy of the transfer mechanism 118 is limited.
Conventional techniques have never recognized such a track pitch accuracy limitation as a problem. However, when the track pitch is decreased to the extent described above, the track pitch accuracy significantly affects the tracking servo. Thus, it becomes very important to suppress degradation of the track pitch accuracy in pursuing a narrower track pitch.